Currently available thermal transfer printing heads generally comprise a plurality of thermal elements. The heating elements are generally provided on one side of flat substrate of ceramic or like material along with the requisite electronic circuitry for controlling the activation of the heating elements.
The thermal transfer printing head is typically used in conjunction with a roller platen assembly and an ink transfer printing ribbon which carries a thermally transferred printing ink. During printing, a web of material to be printed and the printing ribbon are oriented between the thermal elements of the printing head and the roller platen of the roller platen assembly such that the printing ribbon is adjacent to the print head and the material to be printed is adjacent to the roller platen. The roller platen and printing head are moved against each other so that the printing ribbon and the material to be printed are pressed against the printing head such that when selected thermal elements of the printing head are heated, ink from the ink transfer printing ribbon is transferred to the material's surface.
Typical thermal transfer printers can include one or more printing heads each of which are capable of printing in a different color. If more than one printing head is employed, the printing heads are generally arranged in line with each other, with each printing head having a roller platen assembly associated therewith.
In order to produce high quality printing, the proper pressure and thermal energy must be applied by the printing head to the material to be printed and ink printing ribbon. The proper printing pressure is dependent upon many factors including the type of printing ribbon used, the material to be printed, and the width and thickness of the printing ribbon and/or material to be printed. For example, the printing pressure requirements for a waxed based printing ribbon are generally different than the printing pressure requirements for a resin based printing ribbon. The printing pressure requirements would also be different for a paper based material to be printed and a material to be printed comprised of vinyl coated retroflective sheeting.
When the transfer printer includes multiple printing heads and the material to be printed being requires the use of less than all the printing heads, the platen rollers must be moved away from the print heads to preserve the printing ribbon. This is typically accomplished by manually inserting some type of block or like means which moves the roller platen assembly away from the printing heads that are not being used in a particular printing job.
The prior art has attempted to address the issue of printing pressure. For example, U.S. Pat. No. 4,228,441, issued to Rhine entitled PRINTER HEAD BIASING APPARATUS, discloses a cantilevered leaf spring arrangement which maintains a constant and preset pressure between the print head and the material to be printed. The cantilevered spring is adjustable to provide the desired amount of pressure between the printing head and the material to be printed.
U.S. Pat. No. 4,844,632 issued to Minowa entitled HEAD BIASING MECHANISM IN A THERMAL PRINTER, discloses an elastic member for moving the printing head toward and away from the platen. The elastic member urges the printing head against the platen at least two levels of biasing force. The Minowa patent also addresses the issue of printing head disengagement by providing a release mechanism for selectively releasing the biasing force on the printing head.
U.S. Pat. No. 4,949,098 issued to Cluck et al. entitled THERMAL PRINTHEAD CONTROLLING MEANS, discloses a printing head controlling structure for disposing the printing head in a non-printing position and for resiliently supporting the printing head in the printing position.
U.S. Pat. No. 5,206,662 issued to Fox et al. entitled METHOD AND APPARATUS FOR ADJUSTING CONTACT PRESSURE OF A THERMAL PRINTHEAD, discloses a spring mechanism for applying a torque against an arm for rotating a shaft to make printhead contact with the printing ribbon and roller platen. The spring mechanism allows the a wide variety of printer media to be accommodated since the pressure between the platen roller an the printing head is variably adjustable.
U.S. Pat. No. 5,328,281 issued to Narita et al. entitled RECORDING MEDIUM FEED MECHANISM FOR A PRINTER AND METHOD OF MEDIUM FEED CONTROL, discloses a printer for an electronic calculator or electronic cash register. The printer disclosed therein employs solenoids for engaging and disengaging contact pressure rollers which permit the separation or disengagement and contacting or engagement with the printing medium.
U.S. Pat. No. 5,448,281 issued to Walter et al. entitled PRINT HEAD PRESSURE ADJUSTING MECHANISM, discloses a printing pressure adjustment system. The adjustment system employs a steel spring for adjusting the pressure of the printing head.
The mechanisms described above still don't offer the precision necessary to optimize printing pressure for various types of printing ribbons and materials to be printed which are available.
The pressure required for closing of the top door is directly related to the pressure applied on the platen. The pressure required for a reflective media could not be implemented without the necessity of a complex latch assembly.
Accordingly, the is a need for a thermal transfer printer which employs a roller platen assembly having a variably adjustable roller platen pressure that can be adjusted in substantially fine increments in order to optimize the printing process for various types of printing ribbons and printing materials.